This invention relates to scintillation cameras, commonly called gamma cameras, and is particularly concerned with improving nonuniformities or lack of positional correspondence between the distribution of scintillations or radiation events in incremental areas of the camera field and in the displayed image resulting from the scintillations.
A substantial amount of the disclosure herein is similar to that in co-pending U.S. patent application Ser. No. 787,122, filed Apr. 13, 1977 now U.S. Pat. No. 4,115,694. The present application discloses and claims improvements in the invention set forth in the cited application. Both applications are owned by the same assignee.
As explained in the prior application, in nuclear medicine, gamma camera systems are used to detect gamma ray photons emitted from a body in which a radioisotope has been infused. Scintillations occur where photons are absorbed by crystalline material. A typical system is based on the camera of Anger as disclosed in U.S. Pat. No. 3,011,057. As in the Anger camera, the camera disclosed herein comprises an array of photosensitive devices such as photomultiplier tubes, usually hexagonally arranged, having their input ends adjacent a light plate or disk. Beneath the disk is a scintillation crystal which converts incoming gamma photons into light photons or scintillations. A collimator is interposed between the scintillator and the emitting body so that emitted photons will impinge substantially perpendicularly to the planar scintillation crystal.
The array of photomultiplier tubes views overlapping areas of the scintillation crystal. The tubes produce a pulse for each scintillation event. Well-known electronic circuits are used to produce signals representing the x and y coordinates of the scintillations. A pulse height analyzer determines if the pulses are within amplitude limits and, if they are, a z signal is produced which controls a cathode ray oscilloscope display to produce a point of light on its screen at the x and y coordinates corresponding with those of the scintillation event intercepted by the camera. A photographic film may be used as an image integrator of the large number of light spots appearing on the screen of the cathode ray tube. A substantial number of events is required to make up the final picture of radioisotope distribution in the body tissue.
It is known in connection with systems of this type, that if a standard source having uniform isotope distribution is placed close to the crystal and a photograph is made of the image on the display tube, the photograph will show nonuniformity which results from so-called "hot spots" under each photomultiplier tube and "cold spots" between the tubes. The transitions between hot and cold areas are gradual rather than abrupt. In other words, a spot or scintillation event actually occurring between photomultiplier tubes is sensed as being partially shifted under the tubes, causing a decrease in spot density between the tubes and an apparent increase in spot density under the tubes.
The prior application cited above discloses and claims a new scheme for the light spot distribution density in the display to correspond with the actual distribution of gamma ray emissions from the body in incremental areas of the camera field. The prior application discloses a system that is operated in two different modes, a count accumulate mode and a run mode. In the accumulate mode, a reference count is generated and it is used as a basis for determining the number of artificial counts that should be injected in incremental areas of the radiation field to correct for the lack of correspondence between the actual and displayed radiation events. During the accumulate mode, a disc having a radioactive isotope distributed uniformly over its area is placed in the camera field. During this time the scintillation counts for incremental areas in the field are placed in memory locations having coordinates corresponding with the incremental areas or locations where the detectors seemingly detected the scintillation events. When in the system described in the cited application, when one of the memory locations was filled, the accumulate mode was terminated and this left all other memory locations partially filled even though use of a uniform source could be expected to result in all of the locations being completely filled within the same counting time interval.
In the prior application as in this application, following the accumulation mode, the apparatus is switched to operate in the run mode. In the run mode, radiation produced by a body which is infused with a radioisotope produces the radiation field which is detected. During the run mode, the most significant bits of the binary numbers representing the counts in the memory locations obtained during the accumulate mode are constantly compared with the number of counts being obtained in corresponding incremental areas when the body is producing the radiation field. The prior application discloses how to inject just the right number of correction pulses to make up the difference between the number of counts that should be obtained during the run mode and those that are actually being obtained. Thus, each incremental area during the run mode receives a number of correction pulses which is proportional to the difference between the actual count received for the memory location during the accumulate mode and the count that was accumulated in the filled memory location. A unique feature of the system described in the prior application is that the correction pulses were always injected at proper regular intervals while the run mode was in progress as opposed to having all of the correction pulses injected at the end of a run.
Experience with the system described in the prior application demonstrated great improvement in the uniformity of the displayed image over any correcting means that had been used before. However, faint nonuniformities within the bounds of visual acuity could still be discerned. The inventors in this application perceived that the residual problem resulted from over-correction resulting from what was seemingly a truly proportional scheme.